In recent years, a minute fine metal powder having a particle diameter on the sub-micron order composed of various types of metals or alloys has been utilized or considered to be utilized for:                a capacitor, an anisotropic conductive film, a conductive paste, a conductive sheet, and so on by making use of characteristics as a conductive material of the metal or the alloy itself and minuteness of the powder,        a growth catalyst of a carbon nanotube, a reaction catalyst of a gas chemical material, and so on by making use of characteristics as a catalyst material and minuteness, and        an electromagnetic wave shielding material by making use of characteristics as a magnetic material and minuteness.        
As a method of producing such a minute fine metal powder, various producing methods such as a gas phase method for depositing a fine metal powder and making the deposited fine metal powder grow in a gas phase or a liquid phase method for depositing a fine metal powder and making the deposited fine metal powder grow in a solution have been proposed.
For example, Japanese Laid Opened Patent Application No. 11-80816 (1999) discloses a method of producing a fine nickel powder by reducing vapors of nickel chloride within an atmosphere containing sulfur as an example of a producing method by a gas phase method.
As a method of producing a fine metal powder by a gas phase method, a so-called chemical vapor deposition (CVD) method or the like has been generally performed.
On the other hand, Japanese Laid Opened Patent Application No. 11-302709 (1999) discloses a method of producing a fine powder of nickel or its alloy by dropping a solution containing at least nickel ions in a reducing agent solution containing hydrazine, alkali hypophosphite, or alkali borohydride as a reducing agent, to reduce and deposit the nickel ions or the like.
However, sulfur having a concentration of approximately 500 to 2000 ppm is generally contained in a fine metal powder produced by the method disclosed in Japanese Laid Opened Patent Application No. 11-80816 (1999) out of the foregoing methods. Therefore, the purity of the fine metal powder is reduced and correspondingly, characteristics such as conductivity are degraded.
Furthermore, all of conventional gas phase methods, including the producing method disclosed in the above-mentioned gazettes and the CVD method, have the disadvantage that the initial cost and the running cost of a producing apparatus employed for the implementation are significantly high.
Moreover, in the gas phase method, the growth speed of a metal is low. Further, it is difficult to produce fine metal powders in large amounts at one time because the above-mentioned producing apparatus is of a batch type.
Furthermore, in the gas phase method, the growth speed of a metal is low, so that a reaction time period must be made long. Therefore, fine metal powders which are deposited in the early stages of reaction and start to grow and fine metal powders which are deposited later and start to grow greatly differ in particle diameter at the time when the reaction is terminated. Accordingly, the particle diameter distribution of the produced fine metal powders tends to be broad. When an attempt to obtain fine metal powders which are uniform in particle diameter is made, therefore, the fine metal powders whose particle diameters are too large and the fine metal powders whose particle diameters are too small must be removed in large amounts, resulting in significantly reduced yield.
Therefore, the production cost of the fine metal powders produced by the gas phase method are significantly high. In the present circumstances, therefore, the applications thereof are limited.
On the other hand, the liquid phase method can be implemented if there is at least an apparatus for agitating a solution. Therefore, the initial cost and the running cost of the producing apparatus can be made significantly lower, as compared with those in the gas phase method.
In the liquid phase method, the growth speed of a metal is higher than that in the gas phase method. Moreover, it is also easier to increase the size of the apparatus. Therefore, the metal powders can be mass-produced at one time even by a batch-type producing apparatus. Further, fine metal powders can be further mass-produced by employing a continuous-type producing apparatus.
Moreover, a larger number of fine metal powders can be deposited and made to grow uniformly almost simultaneously by making a reaction time period short because the growth speed is high. Therefore, fine metal powders whose particle diameter distribution is sharp and whose particle diameters are uniform can be produced with a high yield.
Out of the above-mentioned methods disclosed in the above-mentioned Japanese Laid Opened Patent Application No. 11-302709 (1999), the method of using alkali hypophosphite or alkali borohydride as a reducing agent has the disadvantage that the purity of fine metal powders to be produced is reduced and correspondingly, characteristics such as conductivity are degraded because phosphorous or boron is deposited together with a metal.
On the other hand, when hydrazine or a hydrazine-based compound is used as a reducing agent, no deposition together with a metal is produced. However, the compound is a hazardous material. Therefore, strict safety management is required for handling.
Therefore, Japanese Patent Application No. 3018655 discloses a producing method using titanium trichloride as a method of producing fine metal powders by a liquid phase method using a new reducing agent which does not have these problems.
That is, in a state where a water-soluble compound of a metal element, together with a complexing agent, as required is dissolved in water to produce a solution, ammonia water or the like is then added as a pH adjuster to the solution to adjust the pH of the solution to not less than 9, titanium trichloride is added as a reducing agent, to reduce and deposit ions of the metal element utilizing the reducing action in a case where trivalent titanium ions are oxidized, thereby producing fine metal powders.
In the above-mentioned gazette, it is advocated that high purity fine metal powders containing no impurities can be produced in safety by such a producing method.
When the inventors have examined the above-mentioned producing method, it has been made clear that the method has the following problems.
(1) In the above-mentioned producing method, fine metal powders having an average particle diameter of approximately 400 nm to 1 μm can be produced. However, minute fine metal powders having a smaller average particle diameter of not more than 400 nm cannot be produced in what way reaction conditions are adjusted.
(2) In a case where titanium trichloride is added to a solution having a pH of not less than 9 in a state where the concentration thereof is 100%, which is not disclosed in the above-mentioned gazette, approximately the whole amount of the added titanium trichloride rapidly reacts with water, to be deposited or precipitated in the solution as titanium oxide by hydrolysis. Even if titanium trichloride is added in the state of a stable hydrochloric acid solution, approximately 20% of the added titanium trichloride reacts with water, to be deposited or precipitated as titanium oxide by hydrolysis. Although in the above-mentioned gazette, it is considered that titanium trichloride is used up at one time, titanium trichloride is difficult to preserve and handle. Moreover, it is high in cost. Therefore, a case where the production cost by the above-mentioned producing method in which titanium trichloride is used up at one time is higher than the unit cost of the fine metal powder to be produced, for example. Therefore, the producing method disclosed in the above-mentioned gazette is not suitable for industrial production of fine metal powders, although a certain degree of results are obtained at a laboratory level.